1. Field of the Invention
Optical communication technology includes wavelength multiplexing techniques which can transmit optical signals with a plurality of different wavelengths simultaneously and allot different information to different wavelengths in order to effectively utilize a transmission path. This type of technique needs optical receivers with a function which enables the separation of optical signals according to their wavelengths. The present invention relates to an optical receiver of this type. More particularly, the present invention relates to an optical receiver which has a function of separating multiplex optical signals according to their wavelengths when the spectrum of the wavelengths of the optical signals is narrow and stable.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional optical receiver. As shown in FIG. 1, a wavelength-multiplexed signal beam 1 is scattered by an angle scatter element 3 after being transmitted through an optical fiber 2; and transmitted light rays are detected by respective photodetectors 4A to 4E which receive corresponding light rays scattered at respective scatter angles. In FIG. 1, reference numerals 5 and 6 indicate an incident side lens 5 and an outgoing side lens 6. As will be clear from the above-described configuration, the conventional optical receiver is a device constructed so as to detect differences in angles in terms of differences in position. Therefore, with the optical receiver, smaller differences in the scatter angle gives rise to a smaller difference in position, which makes the detection of such differences difficult. Generally, an allowance in the difference in position is small, e.g., as small as several tens of micrometers (.mu.m), when the optical receiver is used to divide wavelength-multiplexed light with distances between adjacent wavelengths being on the order of 10 nm, and for this reason, a higher accuracy on the order of micrometers (.mu.m) is required for the optical coupling of the angle scatter element 3 with the photodetector 4. This increases production costs of the device.
In addition, it is natural that the conventional optical receiver deteriorates its demultiplexing characteristics even with a slight fluctuation in the wavelength of the light source because it requires high precision on the order of micrometers in the optical coupling of the angle scatter element with the photodetector.
As will also be clear from the configuration shown in FIG. 1, the variation in wavelengths of the light source results in difference the in the scatter angle for the signal light, which requires alterations to be made in the design of the device. Therefore, the optical receiver lacks sufficient flexibility in its response toward changes in conditions such as fluctuation in wavelength.
As described above, the conventional optical receiver is disadvantageous in that it is uneconomical since its production cost is high and it has poor demultiplexing characteristics as well as it has poor flexibility towards various conditions upon measurement.
As another example of conventional optical receivers there is known a device which has a configuration as shown in FIG., 2. This type of optical receiver includes a combination of an optical receiver with an optical demultiplexer which realizes a Mach-Zehnder interferometer that has been well known in the field of classical optics using optical waveguides.
In FIG. 2, symbols 7A through 7F indicate single mode optical waveguides, 8A and 8B represent directional couplers, 9A through 9D are ports, and 10A and 10B are optical elements. The two directional couplers 8A and 8B are connected to each other via the two single mode optical waveguides 7A and 7B. The optical circuit shown in FIG. 2 has four ports 9A, 9B, 9C and 9D. The two single mode optical waveguides are different from each other in their length. Therefore, considering the case where optical signals are launched into the port 9A, the optical signals which have been separated via the directional coupler 8A and transmitted separately through the single mode optical waveguides 7A and 7B, respectively, are synthesized again in the directional coupler 8B, the optical signal which has been transmitted through the single mode optical waveguide 7A has a phase different from that of the optical signal which has been transmitted through the single mode optical waveguide 7B. The phase difference varies depending on the wavelength (or frequency) of the optical signals. The outputs from the ports 9C and 9D are determined depending on the phase difference.
FIG. 3 shows output characteristics obtained at the ports 9C and 9D, e.g., in the case where the separation ratio is 1:1 assuming that the wavelengths (or frequencies) of the two optical signals launched into the port 9A are f.sub.1 and f.sub.2, respectively. In this case, the optical output at the port 9C is high and that at the port 9D is low at the wavelength f.sub.1 and on the contrary the optical output at the port 9C is low and that at the port 9D is high at the wavelength f.sub.2. Utilizing these characteristics, the device can be used as an optical demultiplexer which allows signals f.sub.1 and f.sub.2 launched into the port 9A to branch out and be outputted from the ports 9C and 9D, respectively. Therefore, in the conventional optical receiver described above, in order to separate individual signals according to respective wavelengths from wavelength multiplexed optical signal, it has been so designed that photodetectors 10A and 10B are connected to the output ports 9C and 9D, respectively, so that electric signals OA and OB obtained from the respective photodetectors can be utilized as they are. That is, the electric signals OA and OB have been utilized as electric signals which correspond to optical signals with wavelengths (or frequencies) f.sub.1 and f.sub.2, respectively.
However, in the case where optical receivers are to be realized with the conventional configuration as shown in FIG. 2, subtle difference in the length and position of the optical waveguides gives a great influence on the wavelength separation characteristics of the devices since they utilize interference between the optical waveguides. For this reason, there has been required advanced techniques for designing and producing optical circuits, and if such techniques are employed the temperature of the optical circuits produced must be controlled with a precision of less than 1/10.degree. C. and thus their characteristics tend to vary absent such temperature control. A further problem is that the configuration of the optical circuit becomes more complex when an increased number of wavelengths is to be used, which causes an increase in the scale of the circuit and deterioration of loss characteristics.